Lighting design
Luminous efficacy
WEBENCH LED Architect
For a long time after I started in engineering, designers maintained paper libraries of data books from chip and component manufacturers and created and debugged their designs with physical circuits on the bench, measuring actual performance with test instrumentation. For really novel and complex projects with groundbreaking performance specs, development cycles executed in that fashion could consume a year or more.
Even in the analog and power realms, product development today needs acceleration from digital tools to meet schedule goals. So for high-volume designs, functional blocks that would once have been implemented from discrete amplifiers, data converters, and passives often are preconfigured for specific applications.
Likewise, even for low-volume custom designs, chipmakers compete not merely on the basis of integrated functional blocks, but also on downloadable or online design tools that cut weeks out of the process of parts selection and circuit optimization—obviously with the intent of selling as many of those application-centric parts into the design as possible.
At this point, those kinds of tools have been upgraded and refined for at least a decade. They’re all darned good, in terms of translating the performance requirements the designer enters into schematics, layouts, and bills of materials (BOMs).
Breaking out of the pack, though, National Semiconductor added several new implementations to its online WEBENCH tool this year. One version fit dc-dc power supplies, another suited LED lighting, and a third targeted powering FPGAs. And in each case, the user interface (UI) stands out.
Shedding Some Light
Let’s look at the LED lighting implementation, because that’s the least familiar application area—to me and, I assume, to many readers. Let’s say you’re faced with an outdoor lighting application (Fig. 1). The design constraints include the number of LEDs per fixture, drive current, and heatsink design, plus the mechanical complexity of the finished product.
There are lots of tradeoffs—for example, the fewer the LEDs you use, the lower the BOM cost, at least for LEDs. That suggests the application of higher-current LEDs, because each LED produces more light. Additionally, you might want to think about driving the LEDs beyond their nominal ratings.
For instance, a “0.35-A” LED can be driven at 0.5 A to get 25% more luminous flux. However, higher operating temperatures accompany higher current. And for the same LED, its luminous flux reduces to 70% of nominal at 125°C, so bigger heatsinks would be needed if you chose to run higher current. The National tool facilitates this kind of design thinking because the WEBENCH database includes full datasheet information and can tell you that for some devices.
There are further design subtleties. For example, each LED’s forward voltage drop varies with both current and temperature. That consideration leads the designer into the mechanics and thermodynamics of heatsinks, compounded with the problem of directionality of the light.
That is, if you were using a light bulb or discharge tube, herding photons in a particular direction would have been more or less a matter of one reflector and one diffuser, and the bulb or discharge tube would help you out thermally by radiating heat in all directions. In contrast, LEDs have to be arrayed to direct the photons where they’re supposed to go.
Thermal management is a different story as well. LEDs don’t work by getting a filament or a plasma excited to a point where it emits photons along with a huge amount of infrared radiant energy. Instead, the heat that LEDs generate comes from losses in the semiconductor junction, just as in a conventional power device. The heat they do generate must be removed in the same way, through the package, and into the environment by way of a heatsink.
All that is just about the LEDs themselves. Don’t forget that you’re driving them with a dc-dc switching supply driven in turns by a mains supply. Simply interconnecting the LEDs further complicates the design. String them in series, and you have a simpler current-balancing problem, but you need a higher voltage applied to each string to accommodate the sum of the individual forward drops. Connect them in parallel, and you lower the required driver output voltage, but you introduce current-sharing issues.
The decisions you make there lead to considerations of driver-topology alternatives: buck for paralleling LEDs, boost, or buck-boost if you wire them in series. But a buck converter is going to need more voltage from the ac supply; boost, more current. (When would you consider a buck/boost? One scenario would involve off-the-grid solar street lighting, which would, of course, require the solar panels to charge batteries during daylight to power the LEDs after dark.)
Building The Tool
The challenge in designing WEBENCH was formidable, according to Phil Gibson, vice president of technical sales tools at National. The tool had to help designers minimize cost, maximize light output, manage heat and safety, and minimize the number of components while optimizing the efficiency of the power train and the efficacy of the lighting (Fig. 2). On top of that, it couldn’t be allowed to create a BOM that couldn’t be sourced in quantities and on a schedule that would support the anticipated production schedule.
That’s what puts this tool a notch above others that just design filters and show you their Bode plots. It requires a GUI that allows for intuitive input while showing results that graphically depict simultaneous tradeoffs on more than two axes (Fig. 3).
How do you do that on an X-Y plot? Pick any two dimensions for X and Y. Then, make the size of the marker (a circle in this case) represent, say, BOM cost. Next, design the interface so that mousing over any circle in the output panel brings up a table of specific data for that design.
Engineers can get finer-grain detail on a narrow range of possible designs by drawing a box around them, which then expands that particular part of the target sector. If they change the axes to represent alternative characteristics, the circles will rearrange themselves. While the software presents all the design choices that meet the input criteria, it also picks the safe, efficient design that best balances the input criteria and highlights it in green.
BOM pricing is dynamic, based on feeds from multiple distributors that represent current pricing for all the third-party passives and magnetics in the various designs. The database is updated several times a day.
In detail, WEBENCH LED Architect supports the design of complete lighting systems up to 100,000 lumens in output. It analyzes hundreds of the latest LEDs from 12 leading manufacturers, 30 heatsinks, National’s own 35 LED drivers, and a library of 21,000 electronic passive components.
National has recently expanded the WEBENCH into other specialized design areas. In September the company introduced WEBENCH FPGA Power Architect for rapid optimization of power supplies for FPGAs.
FPGA Power, Too
As with the LED tool, the expansion required coordination with the manufacturers of the target devices. At this point, the FPGA tool now supports more than 130 FPGAs from Altera. and Xilinx. The challenge with FPGAs lies in the number of power rails and their rapidly changing current demands, along with their demands for sequencing on startup and shutdown. Secondarily, each load may have specific limitations for ripple and noise. Readers can check out the tool at www.national.com/FPGAarchitect.